1. Field of the Invention
The present invention relates to a scanning optical apparatus for scanning a laser beam, and in particular, a scanning optical apparatus for forming an image by use of an optical scanning, for instance, a laser printer, digital copying machine, facsimile device, etc.
2. Discussion of the Background
FIG. 3 illustrates a conventional scanning optical apparatus in which laser light flux emitted from the semiconductor laser, that is, laser diode 1 is transformed (shaped) to a parallel light flux or an almost parallel light flux by use of a collimator lens 2. The laser light flux thus transformed is directed, as incident light, onto the reflection surface of a rotatable polygon mirror 5 through an aperture 3 and a cylinder lens 4, reflected off a reflection surface thereof, and then deflected with the equal angular velocity.
The laser light flux reflected on the surface of the rotatable polygon mirror 5 is compensated to an equal (constant) velocity deflection by use of an optical element 6 having an f property, and then the surface falling-down of the rotatable polygon mirror 5 is compensated by a plastic lens 7 and transformed to a convergent light.
A surface 10 to be scanned is disposed on a beam west position of the convergent light. A reflection mirror 8 and an optical sensor 9 such as photodiode, etc. for receiving the laser light flux reflected by the reflection mirror 8 are arranged at a synchronization detecting position 11 at the side of starting the scanning outside of the effective light path of the optical element 6 and plastic lens 7 in order to detect the laser light flux per one scanning and determine the timing for starting the image writing.
In such structure as shown in FIG. 1, the plastic lens 7 for compensating the surface falling-down of the rotatable polygon mirror is provided for the purpose of obtaining a high density and a high image quality by use of a line beam spot. However, in order to make the plastic lens 7, a material for the long-length lens such as plastic is used for the reasons of manufacturing and cost. A gate for ejecting the plastic is therefore provided in the plastic lens. Furthermore, in such structure, in order to secure surface accuracy and to avoid worsening of birefringence, the effectiveness (functional effect) of the optical effective portion can be secured by putting a constant distance by use of the gate. Furthermore, since the plastic lens 7 is of long-length, an edge portion 7a is usually provided in order secure the strength thereof, and the above-mentioned gate is formed on the edge portion 7a. Generally, the plastic lens 7 is fixedly support on the edge portion 7a.
Furthermore, since the light intensity of the semiconductor laser 1 varies in accordance with the temperature, etc., the light intensity thereof is monitored by a not-shown photodiode (hereinafter, called "monitor diode"). Thereby, the light intensity is controlled to a constant value (APC control). On this occasion, generally, the light-emitting area is a non-imaging area outside of the effective light path as shown in FIG. 4.
Recently, high-speed operation and high density light flux are required. It has therefore been proposed to scan the surface 10 to be scanned with light flux from plural sources, e.g., by employing an LD array including four semiconductor lasers and a single monitor diode, schematically shown in FIG. 4, since it is impossible to perform APC control at the same time for each respective semiconductor laser 1, such that APC control has to be performed by emitting the light sequentially in order.
For this reason, when the intensity of the light flux in the non-imaging area is increased, or otherwise the scanning time for the non-image area is shortened due to high-speed operation, the light emitting time becomes long in the non-imaging area. Furthermore, when the light for performing the APC control is emitted immediately after finishing the image-writing, as shown in FIG. 5, the light flux entering the edge portion 7a at the side of finishing the image-writing by use of the plastic lens 7 is diffused at the edge portion 7a. At this time, the probability of the flare (20a, 20b) occurrence may be increased. That is a problem to be solved. On the other hand, the same light flux enters the optical element 6 and then arrives at the scanned surface 10. That is another problem to be solved.
Particularly, in the recent years, since there is a tendency that the sensitivity of the photosensitive body is improved and thereby the necessary light intensity is decreased, such that even the light power of weak flare exposes the photosensitive body, and thereby, a back stripe or a fogging occurs, resulting in deterioration of the printed image quality.
Japanese Laid-open Patent Publication No. 5-150621 has proposed a structure in which light passes through the f lens and becomes stray light in the black box, and a pair of ribs are respectively provided at the scan starting side and scan ending side located downstream of the f lens in order to intercept the laser light rays directed outside of the printing area. In such structure, although the property of the f lens is not described in the above specification, since there is provided no surface falling-down compensating correcting lens, it seems that the structure has a property of converting the equal (constant) angular velocity deflection to the equal (constant linear) velocity deflection and also a property of compensating the surface falling-down.
However, the above-mentioned background art takes into consideration only the fact that the light passing through the f lens becomes the stray light in the black box and the laser light rays directed outside of the printing area. Therefore, when the light is lit up at the scan ending side of the f lens for performing APC control, the laser light rays reflected on the rotatable polygon mirror are diffusely reflected on the edge at the scan ending side of the f lens. Consequently, flare directed toward the printing area may occur. That is a problem to be solved.
Furthermore, Japanese Laid-open Patent Publication No. 6-3609 proposes a light intercepting member for intercepting the light flux passing through outside of the optical effective area of the optical system between the optical system partly or entirely composed of the plastic lens and the deflection element.
As still another background art, the published specification of Japanese Laid-open Patent Publication No. 6-5920, proposes structure in which it is prevented that the laser light is unnecessarily reflected by the member supporting the mirror for detecting the horizontal synchronization and thereby the photosensitive body is exposed.
However, in all of the above-mentioned background arts, only the light-intercepting at the side of the synchronization detecting element (optical sensor 9), namely, at the image-writing ending side, i.e., the scan ending side, is not taken into consideration. Consequently, in the case of emitting the light from the time of ending the image-writing, flare may occur on the edge portion 7a at the image writing ending side of the lens 7. That is a problem to be solved.